1. Field of the Invention
This invention relates to a laminated sheet of thermoplastic fibers for use in sorbing hydrocarbons and other liquids. In particular, this application relates to a nonwoven fibrous laminate formed by a layer of meltblown thermoplastic material ultrasonically bonded between two layers of needle punched material for providing a composite sheet which exhibits unique sorbent characteristics.
2. Description of the Related Art
Thermoplastics such as polypropylene, polyethylene, polystyrene, polyester, nylon, acrylic and many others can be extruded into very thin fibers, which are then collected and entangled to form a thin, porous nonwoven sheet of material. Such material is usable as an air filter, a filter for certain liquids, electrical and thermal insulation, and as a lining for garments. Some forms of the material sorb oil, tend to repel water, and tend to float on water, and are very useful for picking up oil spilled on a factory floor and for cleaning an oil slick off of a body of water.
One process of extruding and forming thin fibers of polypropylene and other thermoplastics, generally called "meltblowing" is well known and described, for instance, in an article entitled "Superfine Thermoplastic Fibers", Industrial and Engineering Chemistry, Vol. 48, No. 8, pp. 1342-1346, which describes work done at the Naval Research Laboratories; Naval Research Laboratory Report No. 11437, dated Apr. 15, 1954; U.S. Pat. No. 3,978,185 to Buntin et al.; and U.S. Pat. No. 4,100,324 to Anderson et al., each of which is herein incorporated by reference.
One known product produced by the applicant is a two-ply sheet of meltblown polypropylene fibers. This particular known product is made by laminating two layers of meltblown polypropylene fibers and ultrasonically welding the layers together at a number of points uniformly across the length and width so as to form a two-ply composite sheet. This two-ply sheet is further processed into rolls, pads, and wipes for uses such as cleaning an oil spill on a factory floor, wiping off oily machinery, skimming oil off of the surface of a body of water, and for a variety of other applications where the user desires to collect hydrocarbons or other liquids in a convenient, safe and clean manner. During such uses, this two-ply sheet has been found to have greater strength and durability than a single ply sheet of the same thickness.
Several others have made sheets of thermoplastic fiber material for sorbing liquids. For instance, U.S. Pat. No. 4,041,203 to Brock shows a nonwoven material 10 in the form of a laminate having as a top layer an integrated mat 12 of generally discontinuous thermoplastic fibers 14, and as a bottom layer, a web 16 of substantially continuous and randomly deposited filaments 18 of a thermoplastic polymer. The mat 12 and web 16 are attached at intermittent discrete bond regions 24 by passing the material through a pressure nip between heated rolls 42 and 44 having a plurality of raised points. The primary object of Brock is to provide a strong, yet textile-like nonwoven material. As another example, U.S. Pat. No. 4,469,734 to Minto shows a nonwoven web 34 of meltblown polymer fibers having a pattern of apertures 63 formed by passing the web 34 through a nip between heated rolls 67 and 68 engraved with a pattern of raised points. The rolls 67 and 68 rotate at different speeds to both bond the fibers together and to tear the web 34 to make apertures 63 within the bond areas. The apertures provide the web with the capability to absorb large quantities of fluid, especially if the fluid is too viscous to be taken up by the fiber web structure, and to enable the material to readily release the absorbed fluid. U.S. Pat. No. 4,885,202 to Lloyd shows a high strength composite tissue product 25 made by meltblowing a layer of thermoplastic fibers between two layers of paper tissue 7 and 8.
These known thermoplastic fiber sheets, however, do not provide a laminated pad which will draw liquid away from an outer layer and retain the liquid in an inner layer so as to provide the pad with a relatively liquid free outer surface. The prior art further lacks a three-layer pad of thermoplastic fibers in which an inner layer functions essentially as a barrier against liquid passing from one outer layer to the other. Also not known from the prior art is a laminated pad where one layer of material enhances the sorbing and holding capacity of another layer with respect to wicking, retention, contribution and insulation of a spill of hydrocarbons or other liquid.